Abstract
BACKGROUND: The healing mechanism of ruptured or injured tendons is poorly understood. To date, some lineage-specific factors, such as scleraxis and tenomodulin, have been reported as markers of tenocyte differentiation. Because few studies have focused on tenocyte lineage-related factors with respect to the repaired tissue of healing tendons, the aim of this study was to investigate their expression during the tendon healing process. METHODS: Defects were created in the patellar tendons of rats, and the patellae and patellar tendons were harvested at 3 days and at 1, 2, 3, 6, 12, and 20 weeks after surgery. They were studied using micro-computed tomography, and paraffin-embedded sections were then prepared for histological evaluation. Reverse transcription-polymerase chain reactions were performed to analyze the expression of genes related to the tenocyte lineage, chondrogenesis, and ossification. RESULTS: Repaired tissue became increasingly fibrous over time and contained a greater number of vessels than normal tendons, even in the later period. Safranin O staining revealed the existence of proteoglycan at 1 week and its persistence through 20 weeks. Ossification was detected in all tendons at 12 weeks. The expression of tenocyte lineage-related genes was high at 1 and 2 weeks. Chondrogenic genes were up-regulated until 6 weeks. Runt-related transcription factor 2, an osteogenic gene, was up-regulated at 20 weeks. CONCLUSIONS: In our tendon defect model, cells participating in the tendon healing process appeared to differentiate toward tenocyte lineage only in the early phase, and chondrogenesis seemed to occur from the early phase onward. To improve tendon repair, it will be necessary to promote and maintain tenogenesis and to inhibit chondrogenesis, especially in the early phase, in order to avoid erroneous differentiation of stem cells.